Thread rolling and fastener

ABSTRACT

This invention provides a fastener having a shank with a relatively abrupt transition from an unthreaded portion to a full thread dimension, the thread runout occupying no more than one pitch length of the thread in the axial dimension, and circumferentially extending no more than about one-fourth of the circumference of the thread at its pitch diameter. The counterbore of the nut is reduced in length in light of the short transition section of the shank. The threads are rolled on the shank by opposed dies having ridges complementary to the thread to be produced which run out at one edge of the die with spaced, generally symmetrical end walls which are relatively short axially of the ridges so that the full thread dimension is achieved abruptly.

BACKGROUND OF THE INVENTION

The rolling of threads has become accepted as the method for producingsuperior threads for externally-threaded fasteners. However, anunfortunate characteristic of rolled threads is the relatively longrunout at the inner end of the thread where the thread contour is notcomplete. During the thread rolling process, the fastener blank isrolled between two opposed dies having ridges complementary to thethread to be produced. These ridges run out at one edge of the die,which is the location where the inner end of the thread is formed.Inasmuch as the ridges are at a shallow acute angle with respect to thedie edge, a feather edge is produced on these ridges which issusceptible to breakage. Because of this, the ridges of the die aretapered as the die edge is approached, becoming more shallow at the edgeto provide adequate strength at that portion of the die. As a result,two or more turns of the thread at its inner end usually will beincomplete, having less than the full cross-sectional dimension of theremainder of the thread. The incomplete thread can carry no load, andhence does nothing to enhance the performance of the fastener. However,the fastener shank must be made sufficiently long to include the runoutthread at its inner end, as well as the portion of the thread thatcarries the load. Typically, the transition section where the threadrunout occurs has a length axially of the shank corresponding to twicethe pitch of the thread.

Many fasteners include a nut or collar with a counterbore to receive thetransition zone of the bolt or pin that includes the incomplete runoutthread. It is necessary for the counterbore of the nut or collar to havea length sufficient to accommodate the runout thread. Therefore, thelength of both the bolt and nut are dictated by the requirement for theincomplete thread at the inner end of the threaded portion of the bolt.

It has been recognized that making the pin or bolt shorter by reducingthe length of the transition zone with its incomplete threads wouldresult in a saving in weight of considerable significance in theaircraft and aerospace fields, as well as other areas where minimizingweight is critical. Nevertheless, conventional thread rolling will notpermit this.

One approach to reducing the length of the thread runout has been tofirst form a groove in the transition zone to approximately the minordiameter of the thread to be produced. This is accomplished prior torolling the thread, either by cutting to the required geometry or bycutting to less than full dimension, followed by cold rolling to producea groove of the desired depth. After this, the thread is rolled, withthe runout extending into the grooved portion. The result is a threadrunout transition zone shorter than that of a conventional fastener witha rolled thread. A drawback to this system is the increased expenseincurred in the extra operation of forming the groove in the shank priorto the thread-rolling operation.

SUMMARY OF THE INVENTION

The present invention provides a fastener having a significantlyshortened transition thread runout zone, yet without the necessity forproducing a groove in the blank prior to the thread-rolling operation.The thread on the bolt has a relatively abrupt beginning at its innerend, achieving its full cross-sectional dimension much more rapidly thanwith conventional thread-rolling techniques. The inner end of the threadterminates in a wall, preferably rounded concavely, which is generallysymmetrical about the longitudinal axis of the thread. The end wall isrelatively short, extending lengthwise of the thread preferably no morethan one-fourth the circumference of the thread at its pitch diameter.This permits the transition zone at the inner end of the thread toextend axially of the shank a distance no more than a lengthcorresponding to the pitch of the thread. In other words, the transitionzone is about half the length that it is in fasteners with conventionalrolled threads. This permits the counterbore in the nut to be madeshorter because it needs to accommodate only the shorter transition zoneof the bolt. Therefore, there is a weight saving for both the nut andthe bolt.

The ridges on the thread-rolling dies of this invention do not come to afeather edge as they run out on the edge of the die where the inner endof the thread is formed. Instead, they come to relatively abrupt endswhich are generally symmetrical about longitudinal axes of the ridgesrather than being elongated and extremely asymmetrical as in theconventional thread-rolling dies. The ends walls of the ridges areconvexly rounded to produce the rounded inner ends of the threads.

As viewed in elevation, the end walls of the ridges that run out to thedie edge increase very slightly in elevation from the end of the diewhere the blank enters to the end where it leaves. This is to assurethat each ridge end reaches the inner end of the thread groove beingproduced in the blank. The increased elevation causes each die end toengage the blank slightly beyond the groove end as the blank turnsbetween the dies. This is done because it is impossible to assure thateach ridge end will strike the blank in exactly the same place as theblank is rolled between the dies. Therefore, the ridge ends are causedto engage the blank progressively further into the groove to ensure thatthe inner end of the groove is formed properly. The increment of addedengagement at the end of the groove is kept small in order to protectthe dies against breakage.

Fasteners in accordance with this invention are advantageous even whereweight saving is not important. This is because the reduced length ofthe thread runout results in a longer grip length for the bolt. Thismeans that a bolt will have increased versatility by being able tosecure together articles of a greater range of thicknesses. This allowsbolt inventory to be reduced because fewer sizes are necessary toaccomplish a full range of fastening requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a prior art fastener havingrolled threads;

FIG. 2 is a longitudinal sectional view of a fastener made in accordancewith the present invention;

FIG. 3 is an enlarged fragmentary elevational view, showing the innerend portion of the thread of the fastener pin of FIG. 2;

FIG. 4 is a perspective view of the dies performing the thread rollingoperation;

FIG. 5 is an enlarged fragmentary perspective view of one of the threadrolling dies;

FIG. 6 is an elevational view of one of the thread rolling dies and thescrew blank in position for the thread rolling operation;

FIG. 7 is an enlarged fragmentary sectional view, taken along line 7--7of FIG. 6;

FIG. 8 is an enlarged fragmentary elevational view of a portion of oneof the thread rolling dies; and

FIG. 9 is an enlarged fragmentary view illustrating a typical contour ofthe end of the thread-forming ridge of the die.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is a prior art shear fastener 10, commonly used inthe aircraft industry, shown here holding together panels 11 and 12. Thefastener 10 includes a pin 13 having a flush head 14 from which projectsa shank having three sections. The first section 15 of the shankadjacent the head 14 has a straight cylindrical exterior surface of arelatively large diameter. The outer end portion 16 of the shankincludes a rolled thread with a major diameter less than the diameter ofthe section 15. Between the inner and outer portions of the shank is ashorter transition section 17 where the shank tapers from the diameterof the unthreaded section 15 to the smaller diameter outer portion 16.The pin 13 is capable of accommodating panels or other workpieces whosecombined thickness does not exceed the grip length G, which is thedistance from the outer plane of the flush head 14 to the transversegrip plane P at the end of the shank section 15.

The thread extends for the full length of the outer portion 16 of theshank and includes a gradual runout at its inner end portion 18 whichextends into the transition section 17 of the shank. In accordance withconventional practices, the thread is incomplete at the runout portion18, not having its full cross-sectional dimension in that zone. Theportion of the pin 13 occupied by the runout of the thread, therefore,carries no load and cannot perform any useful work as the fastener isused. The length T₁ of the transition section 17 that includes theincomplete threads typically is around twice the pitch of the thread inorder to accommodate the runout portion 18 of the thread.

Engaging the pin 13 is a collar 20 having an intermediate internallythreaded portion 21 that meshes with the thread on the outer end 16 ofthe pin. The base part 22 of the nut 20, which bears against the panel12, flares outwardly and includes a counterbore 23 dimensioned toreceive the transition section 17 of the pin. At the other end, thecollar 20 is unthreaded and includes external wrenching surfaces 24,inwardly of which is an external peripheral groove 25. The latterprovides a frangible portion where the end of the nut can break off uponthe exertion of a predetermined torque as the collar is tightenedagainst the panel.

Illustrated in FIG. 2 is a fastener 27 of the same type as that of FIG.1, but made in accordance with the present invention. The fastener 27 ofthis invention includes a pin 28 and a collar 29 used in securingtogether panels 30 and 31, which are of the same thickness as the panels11 and 12. The pin 28 includes a flush head 32 from which projects ashank having an unthreaded, relatively large-diameter portion 33adjacent the head, a transition section 34 and an outer end portion 35of reduced diameter on which is a rolled thread. The grip length G ofthe fastener pin 28, which is the length of the unthreaded shank portion33 from the head 32 to the grip plane P at the beginning of thetransition section 34, is the same as the grip length G of the fastenerpin 13. The threaded outer end portion 35 also is of a length equal tothat of the threaded end 16 of the pin 13.

The thread on the pin 28 does not have the conventional runout ofincomplete cross section at its inner end characteristic of the threadon the shank end 16. Instead, the inner end portion 36 of the thread hasits full dimension almost to the point where it terminates. This enablesthe transition section 34 to be made shorter than the transition section17 of the conventional fastener pin 13, because it does not have toaccommodate a long thread runout. The length T₂ of the transitionsection 34 may be approximately equal to the pitch of the thread on theshank end 35. This contrasts with the transition section 17 of theconventional fastener which has a length equal to twice the threadpitch. Thus, although the grip length of the fastener pin is the same asbefore, and the threaded section is of equal length to that of theconventional fastener, the overall length of the pin has been reduced bythe distance of one thread pitch. This means that at a longitudinaldistance from the grip plane P corresponding to around one pitch of thethread, the thread will have its full dimension.

The collar 29 also is made shorter than the collar 20 of theconventional fastener. The collar 29 is similar in most respects to thecollar 20, including an outwardly-flaring base portion 38, anintermediate, internally-threaded portion 39, and wrenching surfaces 40on its outer end. A breakaway peripheral groove 41 also is included.However, the counterbore 42 of collar 29 is shorter than that of thecollar 20. This is because the pin transition section 34 is of reducedlength and a shorter counterbore will accommodate it. Accordingly, bothcomponents of the fastener are of reduced length, and a significantweight savings is realized.

Although the inner end of the thread begins abruptly, it is preferred toavoid a flat inner end wall at the terminus of the thread, and insteadto provide a concave, rounded wall 43 of compound curvature. Theresulting transverse end wall, which is generally symmetrical about thelongitudinal axis of the thread, will provide some taper at the innerend part of the thread. This is to improve the life of thethread-rolling dies and to avoid stress risers which could result fromsharp corners in the completed fastener. The resulting runout preferablyis no greater in length than one-fourth the circumference of the pin atthe pitch diameter, and frequently is less.

The thread on the pin 28 is produced by a pair of thread-rolling dies 46and 47, as illustrated in FIG. 4. One of these, the die 46, is movable,while the die 47 is stationary. These dies are flat, but cylindricaldies also can be constructed embodying the principles of this invention.As shown in the drawing, the dies 46 and 47 are used to roll threads onthe outer end part 48 of the shank of a blank 49, which is used toproduce the completed fastener pin 28. The shank of the blank 49 has aportion 50 of larger diameter adjacent its head 51, and a short taperedtransition surface 52 (better seen in FIG. 6) between the end part 48and the portion 50.

The movable die 46, illustrated in enlarged detail in FIGS. 5, 6 and 7is identical in configuration to the stationary die 47. The die 46includes flat parallel longitudinal top and bottom edges 53 and 54, anda vertical face 55 which is used in producing the thread. Formed on theface 55 is a series of parallel ridges 56 which are complementary to thethread to be produced, and therefore generally V-shaped in endelevation. In accordance with standard practice, these ridges includeflanks 57 and 58 with a 60° included angle between them. The ridges 56are at an acute angle relative to the top and bottom edges 53 and 54,appropriate for producing a thread helix when the blank 49 is rolledbetween the dies.

Between the top edge surface 53 of the die and the face 55 is a stepped,beveled surface 59 which forms a part of the upper die edge. Thissurface is at an angle of 25° relative to the top edge surface 53.Accordingly, the stepped surface 59 is at only a 5° differential withrespect to the upwardly-facing flanks 57 of the ridges 56 that run outat the surface 59. In practice, the surface 59 is made to blend with theflanks 57 that it intersects. The vertical dimension of the surface 59should be equal to at least twice the pitch of the thread to be producedto assure adequate clearance as the thread is formed.

The ridges 56 that intersect the stepped surface 59 have relativelyabrupt ends 60 which preferably are convexly rounded, with compoundcurvature. As a result, the ridges 56 have their full cross-sectionaldimension, symmetrical on either side of their longitudinal axes, at alocation close to where they terminate. The only runout of the ridges isprovided by the transverse rounded ends 60, which are generallysymmetrical about the ridge axes. Preferably, this runout does notexceed one-fourth of the circumference of the thread to be produced atits pitch diameter.

The rounded ends 60 of the ridges 58 that extend to the surface 59 blendsmoothly into flat, narrow surfaces 61 which extend to the top edge 53surface of the die. This results in the stepped configuration of thebeveled surface 59, dividing it into segments, each of which connects tothe flank 57 of one of the ridges 56. The surfaces 61 are transversewith respect to the ridges 56, being perpendicular to the longitudinalaxes of the ridges.

Lengthwise of the die, the ridge ends 60 are spaced apart a distance Ethat is approximately the same as the circumference of the thread to beproduced at its pitch diameter. This encompasses some variation from theprecise circumferential distance. For example, the distance E for a dieto produce a fastener pin of titanium may fall within the range ofaround π×0.8×pitch diameter of the thread to π×1.0×pitch diameter of thethread.

The spacing between the ridge ends 60 is made such that there is anincrease in height from one to the next from the die end 62, where thethread rolling begins, to the opposite end 63. This effect isillustrated in FIG. 8, and can be seen in FIGS. 6 and 7, as well. Asshown in FIG. 8, the ridge end 60 on the right (toward the die end 63)is closer to the top die edge surface 53 by a small distance D than isthe ridge end 60 on the left. The same height differential D of adjacentridge ends applies throughout the length of the die.

The dies of this invention are operated as conventional dies, with onedie 46 being moved longitudinally relative to the other die 47. Thescrew blank 49 is positioned prior to the threading operation as shownin FIGS. 4 and 6, which locates it adjacent the end 62 of the die 46 andperpendicular to the die edge surface 53. Therefore, the inner end 43 ofthe screw thread that is produced as the dies are actuated is formed bythe rounded ends 60 of the ridges 56. The result is the relativelyabrupt termination of the screw thread at its inner end, as describedabove.

The gradual increase in elevation of the ends 60 of the ridges 56 fromthe die end 62 to the die end 63 is to make certain that each of theends 60 will strike the screw blank at the inner end of the groove beingproduced to form the thread. This is because it is impossible, as apractical matter, to have each end 60 engage the screw blank atprecisely the same location. Therefore, the gradual increase in heightof the ridge ends 60 assures that each successive ridge engages theblank at a position slightly beyond where the preceding ridge hadengaged it. At the same time, the increase in height is small from oneridge to the next, so that only a small increment of the unformedportion of the screw blank is engaged by the end of the ridge of thedie. This avoids breakage of the ends of the die ridges.

The beveled surface 59 is beneficial in providing clearance at the innerend of the thread being produced, again protecting the ridges of the dieto avoid breakage.

As illustrated, the end surface 60 of each ridge 56 is symmetrical aboutthe longitudinal axis of the ridge in order to produce the symmetricalinner end 43 of the thread. In actual practice, if the ends of theridges are polished by hand to produce the curved end walls 60, theywill not achieve a precise geometric symmetry, as the exposed side ofthe ridge at the flank 57 normally may be cut away a small amount morethan on the side of the flank 58. As used herein, the term "generallysymmetrical" as applied to the end of the ridge and the end of thethread is intended to include such deviations.

Although illustrated with respect to a fastener having three shanksections, the invention can be applied to fasteners having a uniformshank diameter. The inner end of the rolled thread then will terminateabruptly, as in the embodiment described, but in a straight shanksection rather than a tapered transition zone.

The foregoing detailed description is to be clearly understood as givenby way of illustration and example only, the spirit and scope of thisinvention being limited solely by the appended claims.

What is claimed is:
 1. A thread rolling die comprisinga member having aplurality of spaced parallel ridges on one side thereof at an acuteangle to one edge thereof,said ridges being complementary to a thread tobe produced, certain of said ridges having end surfaces adjacent saidone edge of said member which are transverse to the longitudinal axis ofsaid ridges, and said one edge of said member includes a beveled surfaceextending to said certain ridges and a second surface connected to saidbeveled surface, said ends of said certain ridges being progressivelycloser to said second surface from one end of said member to theother,said certain ridges providing a relatively abrupt transition fromsaid end surfaces to the full dimensions thereof for providing arelatively abrupt inner end to a thread being produced by said member.2. A threaded fastener comprisinga member having a head at one end and ashank projecting from said head,said shank including an unthreadedcylindrical first portion of predetermined length adjacent said head, asecond portion of predetermined length at the outer end thereof having arolled thread thereon of uniform dimension throughout the length of saidsecond portion and having a major diameter no greater than the diameterof said first portion, and a third portion interconnecting said firstand second portions and extending axially a predetermined length anddiffering in diameter from said first and second portions, said rolledthread ending within the length of said third portion of said shank asan inner end part which end part has less than said uniform dimensionand extends no more than one-fourth the circumference of said rolledthread at its pitch diameter so as to provide an abrupt end transitionfrom said full dimension to the inner end of said thread, said inner endpart including a generally symmetrical rounded transverse surface abouta longitudinal axis of said thread, said third portion of said shankhaving an axial dimension not exceeding approximately a distancecorresponding to one pitch of said thread.
 3. In a threaded fastener ofthe type havinga member having a head at one end and a shank projectingfrom said head,said shank including an unthreaded cylindrical firstportion of predetermined length adjacent said head, a second portion ofpredetermined length at the outer end thereof having a rolled threadthereon of uniform dimension throughout the length of said secondportion and having a major diameter no greater than the diameter of saidfirst portion, and a third portion interconnecting said first and secondportions and extending axially a predetermined length and differing indiameter from said first and second portions, the improvement comprisingsaid rolled thread ending in said third portion of said shank as aninner end part which end part has less than said uniform dimension andextends rotationally no more than one-fourth the circumference of saidrolled thread at its pitch diameter and which extends axially a distanceno greater than substantially the pitch of said rolled thread so as toprovide an abrupt end transition from said full dimension to the innerend of said thread, thereby to minimize the length and weight of saidfastener, said inner end part including a generally symmetrical roundedtransverse surface about a longitudinal axis of said thread.
 4. Themethod of producing a threaded fastener which is lighter and morecompact than a conventional fastener which has a head and a shankprojecting from said head, which shank includes a first unthreaded partof a first diameter adjacent said head, said first part having a firstpredetermined length and cooperating with said head to define the griplength of said conventional fastener, a second part of a second andsmaller diameter and a second predetermined length at the outer endthereof, said second part having a rolled thread thereon of apredetermined pitch and a substantially constant full dimension for thefull length of said second part, and a transition part of a length equalto approximately twice said pitch of said thread interconnecting saidfirst and second parts, said thread including an elongated tapered innerend part of less than said full dimension in said transition part ofsaid shank, comprising the steps ofproviding a blank having a head, anda shank extending from said head of said blank, with said shank of saidblank having a first part of said first diameter and first predeterminedlength adjacent said head of said blank, a second part of said seconddiameter and second predetermined length at the outer end thereof, and atransition part interconnecting said first and second parts andextending axially a predetermined length and differing in diameter fromsaid first and second parts, and then rolling a thread on said secondpart of said shank of said blank such that said thread has saidpredetermined pitch and a substantially constant dimension for theentire length of said second part of said shank, and simultaneously withsaid rolling of a thread on said second part rolling an innertermination of said thread in said transition part so as to provide anabrupt end of said thread at said inner termination thereof which is ofless than said substantially constant dimension and extends rotationallyfor no more than one-fourth the circumference of said thread at itspitch diameter and which extends axially of said shank a distancesubstantially no greater than the pitch of said thread rolled on saidshank of said blank.
 5. The method of providing a lightweight, compactfastener having a head, and a shank having an unthreaded cylindricalportion adjacent said head, an outer end portion having a rolled threadthereon of a predetermined pitch and predetermined uniformconfiguration, and a transition part between said unthreaded cylindricalportion and said outer end portion having the runout of the inner end ofsaid thread thereon, comprising the steps ofproviding a duality ofdies,each of said dies being provided with a plurality of parallelridges having opposite flanks and having a configuration which iscomplementary in cross section to said predetermined uniformconfiguration, positioning said ridges on each of said dies so as to beat an acute angle with respect to one edge of said die and so thatcertain of said ridges terminate adjacent said edge,one of said flanksof each of said certain ridges being positioned in juxtaposition withsaid edge and the opposite of said flanks of said certain ridges beingremote from said edge, providing a convexly curved generally symmetricalend surface on each of said certain ridges extending between the ends ofsaid flanks, so as to provide an abrupt transition from saidconfiguration to the end of each said certain ridges which does notextend axially of said certain ridges a distance greater than one-fourthof the circumference of said rolled thread at its pitch diameter,providing a blank having a head and a cylindrical shank projecting fromsaid head, with a first section of relatively large diameter adjacentsaid head, a second section of relatively small diameter at the outerend of said shank, and a transition section interconnecting said firstand second sections and axially extending a predetermined length anddiffering in diameter from said first and second sections, the length ofsaid transition section being substantially equal to said predeterminedpitch, then positioning said shank between said dies and relativelymoving said dies so as to cause said ridges to engage and form a threadof said predetermined uniform configuration and predetermined pitch onsaid second section of said blank,with said end surfaces of said certainridges engaging only said transition section to form the end of saidthread therein so as to provide a relatively abrupt change in saidthread from said end of said thread to the full dimension thereof whichextends for less than one-fourth the circumference of said thread at thepitch diameter thereof.